A polymer composition includes a polymer having a glass transition temperature of 120° C. to 400° C. as a simple substance of the polymer and an organic modified layered silicate having a decomposition starting temperature of 190° C. to 350° C. and contained in the polymer. (1) A film including the polymer composition, (2) a gas barrier film including the film and an organic/inorganic hybrid layer obtained by the sol-gel method and provided on the film, (3) a substrate comprising the film or the gas barrier film, and (4) an image display device comprising at least an anode, a light-emitting organic thin film layer and a cathode on the film or the gas barrier film are also disclosed. A polymer composition having both superior heat resistance and superior gas baffler properties, a film comprising the polymer composition, and a gas baffler film having the film, as well as a substrate and image display device exhibiting superior precision and durability are provided.